The use of pressure safety relief valves to protect flow control equipment from damage by unanticipated pressure increases or surges is both well known and accepted common practice. In fact, almost all safety codes require some minimum use of pressure relief valves although the descriptive nomenclature for the pressure relief safety devices employed may vary. Such devices are commonly called "pop" valves, safety valves or relief valves in the art.
Basically, a pressure relief safety valve employs a movable flow closure element that seals with a seat ring fixedly carried by a valve housing. An adjustable spring or other biasing means is typically employed to hold the closure member in sealing engagement with the seat ring. The spring is selected and adjusted to maintain the seal at normal or anticipated fluid pressure operating conditions, but enables the safety valve to open at a higher predetermined or preselected pressure. The predetermined abnormally high valve opening fluid pressure increases the force urging on the spring to compress the spring and separate the closure element from the seat ring to vent the overpressure condition. When the contained pressure drops or returns to the normal operating range, the spring returns the closure element back into sealing engagement with the seat ring in order that normal flow operations and safety protection may be resumed.
To prevent this safety protection against catastrophic failure of the flow control equipment from being intentionally or accidentally disabled, many safety codes normally prohibit the installation of a block valve for isolating the safety relief valves from the operating fluid pressure. This is not entirely satisfactory as the relief valve can only be tested or repaired if malfunctioning only by shutting down and emptying the entire flow system. In some flow applications this may be a massive undertaking that can be accomplished only at great expense.
To overcome this problem, devices and procedures for on-stream or in place testing of safety valves have been developed. In U.S. Pat. No. 4,548,067 to Cox, entitled "Method and Apparatus for Testing a Relief Valve" such an onstream test device and method is disclosed. The Cox device includes a tubular body or housing having an internal bore or flow passage which is placed in internal fluid communication with the operating fluid pressure source or equipment. The relief valve is secured to the housing so that the operating fluid is internally communicated by the housing to the safety valve. Also disposed in the housing bore is an automatic check valve which enables essentially unrestricted flow from the source to the relief valve but prevents or blocks flow from the housing back toward the source. Between the check valve and the safety valve a valved side port is located. Test fluid under pressure is injected into the housing bore through the side port to close the check valve and isolate the test fluid from the operating pressure. The test fluid is communicated to the safety valve for verifying proper operation of the safety valve. By monitoring the pressure of the test fluid at which the safety valve opens the set or opening pressure of the safety valve may be verified. As soon as verification of the opening pressure is completed, the test pressure is vented for automatically restoring the overpressure protection of the safety relief valve. While the disclosed system is a valuable improvement over the prior art, proper operation of the visually concealed internal check valve is not certain. Should the check valve malfunction, the system may be rendered inoperable or worse still, the overpressure protection may be rendered inoperable.
In Webster U.S. Pat. No. 2,178,901 a similar test check valve arrangement is disclosed, but the check valve is located in the safety valve inlet. See also U.S. Pat. No. 3,768,299 to Garren for a test housing which employs a swing-type check valve.
Simonson et al U.S. Pat. No. 2,952,151 discloses as specialized external tester for an internal safety valve. Such internal safety valves do not lend themselves to onstream or in service testing.
Sebring et al U.S. Pat. No. 3,269,170 discloses a test apparatus for relief valves. The apparatus determines the valve operation pressure prior to the valve "popping" to relief pressure and therefor prevents loss of operating fluid.
U.S. Pat. No. 3,872,875 to Raidl, Jr. discloses use of a frangible diaphragm or rupture disc that normally isolates the safety valve inlet from the operating fluid. A side port for venting and enabling testing of the safety valve is located between the reverse buckling rupture disc and the safety valve. During testing the rupture disc isolates test fluid from the operating fluid. During pressure relief operation, the rupture disc fails by buckling to enable operation of the safety valve.
An "Apparatus For Periodically Testing The Operation Of Safety Valves" is disclosed in Trevisan U.S. Pat. No. 4,428,223. The calibrated spring of the safety valve is tested by applying an external force to operate the valve. From the force values obtained by testing, the opening pressure of the safety valve may be calculated without actually opening the safety valve.
Vanderburg U.S. Pat. No. 4,480,653, is entitled "In-Service Test Valve." The test valve is shiftable automatically in response to a test pressure between an in-service position or condition and a test position that separates in-service and test pressure. The operation of the safety valve disclosed for use with this valve employs a control sensor to supply a back pressure to the dome of the safety relief valve for holding or biasing that valve in the closed position. The test pressure is applied to the control sensor while the dome of the safety valve is isolated from the test pressure.
Each of the above mentioned patents is hereby totally incorporated by reference for forming a portion of this disclosure as if they were fully set forth herein.